You would have invested in the capital gain account scheme where you would have mentioned that you will purchase property within 2 years and now if you decide to convert it into under construction purchase then you need to shift your money.
Was it just a land or house? Are you claiming deduction u/s 54 or 54F?
Secondly if you get a possession certificate from the builder till october 2019 then also it will be considered as you have purchased the property as there are various case laws which suggest that if you have got the possession of a property but there are some small work left it will qualify as valid purchase.
You can ask your builder whether he is going to opt for new gst rate or old one if he opts for new scheme then you might save some amount. If the flat is still not ready to move then you will have to either let go GST or capital gain tax.
Have you not booked the flat yet?
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